In a conventional fiber coaxial network (HFC) of a Community Antenna Television (CATV), TV programs from the front are transmitted to optical nodes around subscribers in a TV network via fibers (under normal circumstances, an optical node covers 300 to 500 subscribers around). TV signals are converted from optical signals to electrical signals at the optical node, and then transmitted to residents' homes through a coax distribution network and via residential buildings.
Please refer to FIG. 1, which is a diagram illustrating the structure of a network from the access point to coaxial terminals in a conventional CATV coax distribution network. As shown in FIG. 1, the system includes an Ethernet access point, 36 splitters (e.g., #11 and #61) and 72 coaxial terminals (e.g., #111 and #612). The television signal output from the Ethernet access point, after being amplified by a building amplifier, passes through a six-branch distributor which distributes the energy of the television signal evenly to six units of the building, and then at each floor of each unit, the television signal is distributed to two residential homes (i.e., two coaxial terminals) through a two-branch splitter.
With the growth of demand on new bidirectional transmission services (for example, interactive digital television, broadband services such as data, voice, video and other multimedia communications implemented in a CATV network), the CATV network, which is only capable of transmitting signals in one direction, needs to carry out bidirectional services. A primary problem encountered is bidirectional reconstruction, which is a threshold to be stride when the CATV network is required to develop from a single function to multi-functions and from a radio and television network to an information network.
At present, a typical application is a bidirectional HFC network obtained by bidirectional reconstruction, which is realized by an asymmetrical frequency division for uplink and downlink. At the front, various service signals, such as analog television signals, digital video signals, computer data signals, telephone signals and various control signals etc., are modulated into different channels of downlink frequency segments in a Sub-Carrier Multiplexing (SCM) mode, and then are transmitted to an optical node via optical fibers after an electronic-to-optic conversion, and the service signals at the optical node are transmitted to subscribers via a coax in a broadcasting mode after an optic-to-electronic conversion. Subscribers' uplink signals are multiplexed into an uplink physical transmission channel by adopting a multiple access technology (e.g., Frequency Division Multiple Access, FDMA for short) and are transmitted to an optical node via a coax for an electronic-to-optic conversion, and then the uplink signals are transmitted to the front via optical fibers. However, the costs of such bidirectional reconstruction are relatively high overall, and the average costs of each residential home's reconstruction are about 300 Yuan. In addition, the major costs are on the bidirectional reconstruction of data transmission implemented on the last 100 meters near the end subscriber in the coax distribution network.
As well known, Ethernet has advantages of being simple, low cost and easy to extend, and if it is possible to apply the present mature Ethernet bidirectional data transmission technology directly to the existing coax distribution network, the costs of the reconstruction for bidirectional data transmission in the coax distribution network can be reduced significantly without doubt. However, the coax distribution network is in a point-to-multipoint physical/logical topology structure while the conventional Ethernet is based on a point-to-point protocol and adopts a base-band transmission manner, therefore, if the Ethernet is adopted to implement the reconstruction for bidirectional data transmission on the last one hundred meters of the existing HFC networks, it is necessary to solve the following three technical problems:
1) The existing physical layer technique is to be improved, i.e., without changing the architecture of the existing CATV coax distribution network, to make a signal source of an Ethernet access point communicate with each coaxial terminal while the coaxial terminals not communicate with each other.
2) How to adopt the Ethernet technology to transmit a point-to-point Ethernet protocol in a point-to-multipoint coax distribution network.
3) The same physical transmission channel is adopted to transmit both downlink data and uplink data.
Regarding the second problem mentioned above, the technical problem of how to transmit the point-to-point Ethernet protocol in the coax distribution network having a point-to-multipoint physical/logical topology can be resolved successfully if it is possible to apply architecture of an Ethernet over Passive Optical Network (EPON for short) in the coax distribution network.
The EPON system is a bidirectional dual-fiber optical access network adopting a Point-to-Multipoint (P-to-MP for short) structure, which includes an Optical Line Terminal (OLT for short) at network side, Optical Network Units (ONU for short) and an Optical Distribution Network (ODN for short). And EPON is located between a Service Network Interface (SNI for short) and a User Network Interface (UNI for short), and is connected with a user device through the UNI.
Please refer to FIG. 2, which is a diagram illustrating the architecture of an EPON system. As shown in FIG. 2, a typical EPON system consists of an OLT, a plurality of ONUs (ONU #1, ONU #2, . . . , and ONU #n) and a Passive Optical Splitter (POS for short). The OLT is located in a Central Office (CO for short) at network side, and the ONU is located in a corridor or a user's home, where the OLT and the ONU is connected with each other via a POS. The POS is adapted to distribute downlink data and collect uplink data.
According to the EPON technology, data are transmitted in a broadcasting mode on the downlink and in a time division multiplexing mode on the uplink. The uplink and the downlink belong to different optical-fiber physical transmission channels, and data transmission on the uplink and that on the downlink can be performed simultaneously. In a downlink direction (from an OLT to ONUs), the signal sent from the OLT reaches each ONU via a 1:n passive splitter (or via several cascade splitters). And in an uplink direction (from an ONU to an OLT), the signal sent from one ONU can reach the OLT only and can not reach any other ONU.
In the physical (PHY) layer of the EPON, 1000 BASE Ethernet PHY is adopted, and at the same time, new Media Access Control (MAC for short) commands are added to the EPON transmission mechanism in order to control and optimize the bursting data transmission and real-time Time Division Multiplex and Multiplexer (TDM for short) communications between each ONU and the OLT. In the second protocol layer of the EPON, the mature full-duplex Ethernet technology is adopted as well as the TDM technology, where no conflict will happen and Carrier Sense Multiple Access/Conflict Detection (CDMA/CD) is not necessary since the ONU sends packets within its own time slot, and thereby the bandwidth is fully utilized.
Specifically, the EPON, compared with the traditional Ethernet, mainly further includes two functions: a simulation sub-layer located under the MAC layer and Multi-Point Control Protocol (MPCP for short) which is regarded as a part of the MAC layer.
Please refer to FIG. 3, which shows a relationship between EPON protocol layers and an Open System Interconnect (OSI) reference model. A simulation sub-layer makes the point-to-multipoint network below look like a plurality of point-to-point links when seen from an upper protocol layer, which is achieved by adding a Logic Link Identity (LLID) as a substitute of a two-byte leading code at the beginning of each group. In the EPON, an optical signal can be accurately transmitted to the end user, and the data from the end user can be transmitted to the central network without adopting any complicated protocol. The MPCP control protocol acts as an extension of the MAC control sub-layer in order to support the normal operation of the communications between the OLT and multiple ONUs in the EPON system, where the ONUs' MAC addresses are identified by their respective LLIDs which are dynamically allocated in their registration procedure.
The MPCP control protocol provisions three procedures:
1) A Discovery Procedure:
Please refer to FIG. 4, which shows a flow chart for discovering an ONU and completing the ONU's registration according to the MPCP control protocol. As shown in FIG. 4, when the system has just been on power, an OLT sends a broadcasting message, all of the ONUs having been on power will receive the message, and each ONU receiving the message will send to the OLT a message to inform the OLT its own information such as ID etc. The OLT will know which ONUs in the system are in an on-power working state on after receiving the registration messages sent from all the ONUs on power.
2) A Report Processing Procedure:
This processing procedure is to complete the collection and generation of all kinds of report information via an uplink bandwidth request sent from the ONU to the OLT. Specifically, as each ONU will report which traffic is to be transmitted, the OLT can know the status of all the ONUs that wait for traffic transmission, and after comprehensive consideration and scheduling, the OLT will allocate a corresponding transmission time slot for each ONU via the following threshold processing message and guarantee that the transmission time slots of multiple ONUs do not conflict with each other. And thus, the multiple ONUs is able to share the same uplink physical transmission channel without any conflict between each other.
3) A Threshold Processing Procedure:
This processing procedure is to accomplish the collection and generation of management threshold information through multi-path transmission of a multiplexer. Specifically, the OLT allocates different transmission time slots to multiple ONUs, and each ONU sends out traffic data that to be transmitted to the OLT at the designated time slot allocated by the OLT. In this way, more than one ONU is able to share the same uplink physical transmission channel without any conflict between each other.
To sum up, according to the above EPON MPCP protocol, a physical transmission channel is in use when an OLT sends data (downlink) to an ONU while another physical transmission channel is in use when the ONU sends data (uplink) to the OLT. For example, an uplink channel is an optical fiber while a downlink channel is another optical fiber, and the two physical transmission channels are two separate physical optical fibers, thereby no interference exists between them and they can be in use simultaneously.
However, compared with the EPON system adopting a bidirectional dual-fiber optical access network in a point-to-multipoint structure, in the point-to-multipoint physical and logical topology of the access point and coaxial terminals in a coax network environment, the same physical transmission channel is adopted to transmit uplink data and downlink data. Therefore, the problem of the uplink and the downlink sharing one physical transmission channel is necessary to be resolved if the MPCP protocol is required to be applied in the coax network environment.